1. Field of the Invention
The present invention relates in general to evaporative emission control systems for automotive vehicles and more particularly to an apparatus for determining if a leak is present in an evaporative emission control system for an automotive vehicle.
2. Description of the Related Art
An evaporative emission control system includes a canister containing activated charcoal to collect and store volatile fuel vapors from a fuel tank during the time the engine is not running. The evaporative emission control system also includes a purge line or conduit connecting between an intake pipe portion downstream of a throttle valve and the canister. The purge line opens under a predetermined condition after start of the engine to draw fresh air into the canister and purge the canister. The collected volatile fuel vapors are thus drawn from the canister into the intake pipe, for combustion within a combustion chamber of the engine.
In this instance, if a fuel vapor flow passage extending from the fuel tank to the intake pipe has a leak or the fuel vapor flow passage has a connecting portion of which seal is defective, the fuel vapors are released to the atmosphere. To prevent such evaporative fuel emission, a diagnostic system has been proposed to determine if a leak is present in the evaporative emission control system, as disclosed in Japanese provisional patent publication No. 7-139439. A leak of the above described fuel vapor flow passage can be checked by closing the passage so that the passage is in the form of a closed space, i.e., by closing the passage in a way as to prevent fluid communication between the inside and the outside of the passage, and observing a variation of the internal pressure of the fuel vapor flow passage after the passage is pressurized in such a way that the internal pressure of the passage and the atmospheric pressure differ relatively firm each other. The diagnostic system of the above described publication thus includes a vent control valve provided to an atmospheric vent of the canister to selectively open and close the atmospheric vent. The atmospheric vent of the canister is closed by the vent control valve when the above described passage is to be closed so as to be in the form of a closed space. The diagnostic system also includes a pressure sensor provided to the above described fuel vapor flow passage for checking a pressure variation of gas enclosed in the passage. A negative pressure produced in the intake pipe portion downstream of the throttle valve is introduced into the fuel vapor flow passage for negative pressurization thereof, whereby to check if a leak is present in the passage.